Screw caps of this type, in which the security ring is resiliently deformable, are already known. The security ring is designed to be forced over and past a collar arranged around the neck of a container to be sealed, when the cap is screwed onto the neck of the container for the first time. The security ring, collar and bridge portions are shaped and dimensioned so that when, after the cap has been fully screwed on for the first time, it is subsequently unscrewed, the security ring is retained by engagement with the collar and the bridge portions are broken as the cap portion is unscrewed.
In previously known screw caps of this type, it has been extremely difficult to shape and dimension the bridge portions and to devise their attachment to the cap-portion and/or sealing ring so that the bridge portions do not break when the cap is screwed on for the first time, but break easily when the cap is subsequently unscrewed.
U.S. Pat. No. 3,455,478 describes a screw cap of this type in which an attempt is made to solve the latter problem by inclining the bridges forwards and downwards, where the cap is considered as being turned forwards and downwards to screw the cap onto the respective container neck. The bridges are thus subjected to compression loading when the cap is being screwed on for the first time and to loading in tension when the cap is being screwed off for a first time. The aim is to utilize the feature of certain materials, particularly plastics, of withstanding compression loading better than loading in tension. A problem with the screw cap shown in this U.S. patent is that the bridges must be made extremely thin and narrow at their attachment points to the lower part or skirt of the cap portion, in order to permit the cap to be screwed off with reasonably little exertion of force and at the same time achieve rupture at the points of attachment of the bridge portions. However, such thin, narrow attachment points are subjected to considerable shearing forces when the cap is screwed on and sealed in the normal way. This necessitates the use of a complicated machine, which grips both cap portion and security ring.